1. Field of the Invention
This invention relates to an image reproducing method and apparatus for reproducing a visible image from an image signal, which is obtained from a color image carried on a reflection type of image storage sheet, such as a photograph or printed matter, or a transmission type of image storage sheet, such as negative film or reversal film.
2. Description of the Prior Art
Recently, research of digital photo printers has been carried out. With the digital photo printers, an image having been recorded on photographic film (hereinbelow referred to as the film), such as negative film or reversal film, or on printed matter is photoelectrically read out, and an image signal having thus been obtained is converted into a digital signal. The digital signal is then subjected to various kinds of image processing, and a processed image signal is thereby obtained. Thereafter, recording light is modulated with the processed image signal, and a photosensitive material, such as photographic paper, is scanned with and exposed to the modulated recording light. In this manner, a visible image is printed on the photosensitive material.
With the digital photo printers, layouts of printed images, such as combining of a plurality of images, division of an image, and editing of characters and images, and various kinds of image processing, such as color/image density adjustment, conversion of magnification, and contour emphasis, can be carried out freely. Therefore, prints having been edited and processed freely in accordance with applications of the prints can be obtained. Also, in cases where the conventional surface exposure techniques are employed, the image density information having been recorded on film, or the like, cannot be reproduced perfectly due to limitation imposed upon the reproducible image density range of photosensitive materials. However, with the digital photo printers, prints can be obtained such that the image density information having been recorded on film, or the like, can be reproduced approximately perfectly.
Basically, the digital photo printers are constituted of a read-out means for reading out an image having been recorded on an image storage sheet, such as film, and an image reproducing means. The image reproducing means carries out image processing on the image signal having thus been detected by the read-out means, and adjusts exposure conditions. Also, the image reproducing means carries out a scanning exposure operation on a photosensitive material under the adjusted exposure conditions and carries out development processing on the exposed photosensitive material. Further, the image reproducing means can reproduce a visible image from the image signal having been obtained from the image processing and can display the visible image on a monitor.
For example, in a read-out apparatus for reading out an image having been recorded on film, or the like, wherein slit scanning is carried out, reading light having a slit-like shape extending in a one-dimensional direction is irradiated to the film, and the film is moved in a direction, which is approximately normal to the one-dimensional direction. (Alternatively, the reading light and a photoelectric converting device are moved in the direction, which is approximately normal to the one-dimensional direction.) In this manner, the film is scanned in two-dimensional directions. An image of the light, which has passed through the film and carries the film image information, is formed on a light receiving face of the photoelectric converting device, such as a CCD line sensor, and is thus photoelectrically converted into a light amount signal. The thus detected light amount signal is amplified and is then converted into a digital signal by an analog-to-digital converter. Thereafter, the digital signal is subjected to various kinds of image processing, such as compensation for a fluctuation in the characteristics among the CCD elements of the CCD line sensor, image density conversion, and conversion of magnification, and a processed signal obtained from the image processing is transferred to a reproducing means.
In the reproducing means, for example, a visible image is reproduced from the received image signal and displayed on a display device, such as a cathode ray tube (CRT) display device. When necessary, the operator, who views the reproduced image, corrects the gradation, the color, the image density, or the like, of the reproduced image (i.e., sets the set-up conditions). In cases where the reproduced image is judged as being acceptable as a finished print, the image signal is transferred as the recording image information into a development means or a monitor.
In an image reproducing apparatus, in which the image reproduction with raster scanning (i.e., light beam scanning) is utilized, three kinds of light beams corresponding to exposure of the layers, which are formed on a photosensitive material and are sensitive to three primary colors, e.g. red (R), green (G), and blue (B), are modulated in accordance with the recording image information and deflected in a main scanning direction (which corresponds to the aforesaid one-dimensional direction). Also, the photosensitive material is conveyed in a sub-scanning direction, which is approximately normal to the main scanning direction. (The photosensitive material is thus moved with respect to the deflected light beams and in the sub-scanning direction.) In this manner, the photosensitive material is scanned in two-dimensional directions with the light beams, which have been modulated in accordance with the recording image information, and the image having been read out from the film is thereby reproduced on the photosensitive material.
The photosensitive material having thus been scanned with and exposed to the light beams is then subjected to development processing in accordance with the kind of the photosensitive material. For example, in cases where the photosensitive material is a silver halide photographic material, it is subjected to the development processing comprising the steps of color development, bleach-fix, washing, drying, and the like. A finished print is thereby obtained.
Such a photosensitive material can record a comparatively wide range of luminance of the object. However, the maximum image density on the photosensitive material is limited. Therefore, in cases where a print of a scene having a large difference in luminance is obtained with an ordinary printing technique, details become imperceptible due to insufficient gradation in either one of a bright portion (a highlight) and a dark portion (a shadow) on the print. For example, in cases where a picture of a person is taken against the light, if the picture is printed such that the image of the person may become clear, the bright portion, such as the sky region, will become white and its details will become imperceptible. Also, if the picture is printed such that the bright portion, such as the sky region, may become clear, the image of the person will become black and its details will become imperceptible. In order to solve the problems, a shutting light technique or a masking print technique has heretofore been employed.
With the shutting light technique, an ordinary level of exposure is given to a region having an intermediate level of image density in a scene. Also, a long time of exposure is given selectively to a region, which is considered to become white and the details of which are considered to become imperceptible on the print, by using a perforated blocking sheet. Further, as for a region, which is considered to become black and the details of which are considered to become imperceptible on the print, the exposure time is kept short selectively by using a blocking sheet. In this manner, the print is obtained such that the contrast of each object may be kept appropriate, and the details of the highlight and the shadow may be kept perceptible. A method has been proposed, in which unsharp image film having been photographically formed by the negative-positive reversal of original image film is used as a blocking sheet for locally controlling the exposure time, and in which the printing is carried out by superposing the original image film and the unsharp image film one upon the other.
Also, various masking print techniques have been proposed in, for example, Japanese Unexamined Patent Publication Nos. 58(1983)-66929 and 64(1989)-35542 and Japanese Patent Publication No. 64(1989)-10819. With the proposed masking print techniques, the same effects as those of the shutting light technique can be obtained by locally changing the brightness of a light source for illuminating a photographic original image. With the apparatus proposed in Japanese Unexamined Patent Publication No. 58(1983)-66929, a CRT is employed as the illuminating light source, and a photometric operation with memory scanning is carried out on an original image. In this manner, an unsharp mask signal is formed from the color original image. In an exposure mode, a CRT is controlled with the unsharp mask signal, and the contrast is thereby controlled such that the original image may be reliably recorded within the contrast reproduction limit of a photosensitive material.
With the apparatus proposed in Japanese Unexamined Patent Publication No. 64(1989)-35542, a CRT is employed as the illuminating light source, and an optical path for the photometric operation carried out on an original image and an optical path for the exposure of a photosensitive material are provided such that they may be changed over to each other. Also, a signal for controlling the luminance of the CRT during the exposure and thereby correcting the gradation and the saturation of the reproduced image is formed in accordance with the photometric signal obtained from the original image. Further, a signal for displaying the reproduced image on a monitor is formed. The image displayed on the monitor is viewed, and the amount of light of the CRT is thereby controlled such that a desired image may be reproduced.
With the apparatus proposed in Japanese Patent Publication No. 64(1989)-10819, a matrix device, such as a liquid crystal, which is capable of locally changing the light transmittance, is located between a uniform surface light source and an original image. The transmittance of the liquid crystal is controlled in accordance with the photometric signal obtained from the original image, and the contrast of the reproduced image is thereby adjusted.
Further, for example, in Japanese Unexamined Patent Publication No. 6(1994)-242521, a method is proposed wherein, in order for the gray balance in image reproduction to be corrected, conversion is carried out such that the maximum image density value and the minimum image density value of each color on an original image may become equal to predetermined values on the reproduced image. With the proposed method, the control of the gradation can be carried out for each of the frames of film. Therefore, as for a scene having a large difference in luminance, the gradation of the entire area of the image can be rendered soft such that the range of luminance of the scene may fall within the dynamic range of the photosensitive material. In this manner, the problems can be prevented from occurring in that the details of the highlight and the shadow become imperceptible due to insufficient gradation.
However, with the aforesaid shutting light technique and the aforesaid masking print techniques, the blocking sheet having been prepared regardless of the image, which is to be reproduced, must be operated. Therefore, a very high level of operation technique is required. Also, considerable labor and time are required to form the unsharp image film, and the printing efficiency cannot be kept high.
Also, with the aforesaid apparatuses described in Japanese Unexamined Patent Publication Nos. 58(1983)-66929 and 64(1989)-35542 and Japanese Patent Publication No. 64(1989)-10819, the contrast of a comparatively large structure can be reproduced by adjusting with the distribution of the luminance of the illuminating light source. However, local structures in the reproduced image correspond to the projected image of the original image film. Therefore, the aforesaid apparatuses have the drawbacks in that the reproduction of colors of the local structures, including their edges, cannot be controlled freely, in that the sharpness of the edges cannot be controlled freely, and in that the gradation of over-exposure portions, under-exposure portions, or the like, in the original image cannot be controlled freely.
Further, with the aforesaid apparatuses described in Japanese Unexamined Patent Publication Nos. 58(1983)-66929 and 64(1989)-35542 and Japanese Patent Publication No. 64(1989)-10819, the processing for the photometric operation and the exposure is carried out sequentially. Therefore, the problems occur in that the processing capacity cannot be kept high. Also, the problems occur in that, in cases where the distance of movement of the original image becomes different between when the photometric operation is carried out and when the exposure is carried out, the printed image becomes disturbed. Further, with the apparatus described in Japanese Patent Publication No. 64(1989)-10819, wherein the liquid crystal is used, since the transmittance of the liquid crystal is at most approximately 30%, the exposure time cannot be kept short. Furthermore, the tube surface of the CRT is covered with glass, and the side inward from An the glass becomes luminous. Therefore, even if the film is brought into close contact with the tube surface of the CRT, a spacing will substantially occur between the luminous surface of the CRT and the film. Accordingly, with the apparatus proposed in Japanese Unexamined Patent Publication No. 64(1989)-35542, wherein the image represented by the photometric signal is displayed, a blur occurs with the photometric and image forming system due to the spacing between the luminous surface of the CRT and the film surface during the photometric operation, and therefore a clear monitor image cannot be obtained.
With the method proposed in Japanese Unexamined Patent Publication No. 6(1994)-242521, even though the problems can be prevented from occurring in that the details of the highlight and the shadow become imperceptible due to insufficient gradation, the problems occur in that the contrast of each object becomes weak and the printed image becomes monotonous.
Therefore, novel image reproducing methods have been proposed, wherein an unsharp image signal, which represents only the structures of low frequencies in a color image, is subtracted from a digital image signal representing the color image, a difference signal being thereby obtained, processing for changing the image density, the saturation, and/or the gradation is carried out on the difference signal, and a visible image is reproduced by an image reproducing means from a processed image signal, which has been obtained from the processing carried out on the difference signal. In this manner, even if the contrast of the entire area of the original image is strong, a reproduced image can be obtained such that the contrast of the entire area of the image may be weakened, such that the contrasts of fine structures in the highlight and the shadow in the image may remain, and such that the details of the highlight and the shadow can be prevented from becoming imperceptible in the reproduced image due to insufficient gradation. Such image reproducing methods are described in, for example, Japanese Unexamined Patent Publication No. 2(1990)-226375 and U.S. Ser. No. 08/672,939.
However, with the proposed image reproducing methods, even though the dynamic range of the entire image can be compressed, the problems occur in that the dynamic range compressing process is carried out also on a scene, in which the large area contrast is weak and which need not be subjected to the dynamic range compression. Therefore, the entire area of the image becomes monotonous, and the image becomes imperceptible.
Also, several scenes have the characteristics such that the histogram of the image signal representing the scene is biased to the bright side or the dark side. As for such scenes, the dynamic range compressing process should preferably be carried out on only either one of the highlight and the shadow. However, with the conventional methods, the dynamic range compressing process is carried out uniformly over the entire area of the image, and therefore the image portion, in which the large area contrast is weak, becomes monotonous, and an image having a high perceptibility cannot be obtained.
The primary object of the present invention is to provide an image reproducing method wherein, even if an original image has a strong low frequency contrast, the details of a highlight and a shadow are prevented from becoming imperceptible in a printed image due to insufficient gradation, the printed image is prevented from becoming monotonous, adverse effects of a dynamic range compressing process are prevented from occurring on a scene having a weak low frequency contrast, and the printed image having good image quality is thereby obtained.
Another object of the present invention is to provide an image reproducing method, wherein the color reproducibility in a printed image is enhanced such that an unnatural feeling may not occur at portions in the vicinity of edges in the printed image, and wherein the printed image having good image quality is thereby obtained even from an original image having a strong large area contrast.
The specific object of the present invention is to provide an apparatus for carrying out the image reproducing method.
The present invention provides an image reproducing method, wherein a visible image is reproduced from a digital image signal representing a color image, the method comprising the steps of:
i) forming a histogram of the image signal,
ii) calculating a dynamic range of the image signal in accordance with the histogram,
iii) setting a rate of dynamic range compression corresponding to a region of the image signal, in which the signal value is comparatively large, a region of the image signal, in which the signal value is comparatively small, and/or the entire region of the image signal, the rate of dynamic range compression being set in accordance with the dynamic range and by taking a predetermined reference level of the image signal as reference (such that the rate of dynamic range compression may become equal to approximately zero with respect to the reference level), .
iv) carrying out a dynamic range compressing process on the image signal with the rate of dynamic range compression having thus been set,
v) carrying out gradation processing on an image signal, which has been obtained from the dynamic range compressing process, a processed image signal being obtained from the gradation processing, and
vi) reproducing a visible image from the processed image signal.
The present invention also provides an image reproducing apparatus, wherein a visible image is reproduced from a digital image signal representing a color image, the apparatus comprising:
i) a histogram forming means for forming a histogram of the image signal,
ii) a dynamic range calculating means for calculating a dynamic range of the image signal in accordance with the histogram,
iii) a compression rate setting means for setting a rate of dynamic range compression corresponding to a region of the image signal, in which the signal value is comparatively large, a region of the image signal, in which the signal value is comparatively small, and/or the entire region of the image signal, the rate of dynamic range compression being set in accordance with the dynamic range and by taking a predetermined reference level of the image signal as reference,
iv) a dynamic range compressing process means for carrying out a dynamic range compressing process on the image signal with the rate of dynamic range compression having thus been set,
v) a gradation processing means for carrying out gradation processing on an image signal, which has been obtained from the dynamic range compressing process, and thereby obtaining a processed image signal, and
vi) a reproducing means for reproducing a visible image from the processed image signal.
In the image reproducing method and apparatus in accordance with the present invention, the dynamic range a compressing process should preferably be carried out only when the dynamic range is larger than a predetermined threshold value.
Also, the image reproducing method and apparatus in accordance with the present invention should preferably be modified such that a preliminary read-out image signal, which represents picture elements detected at coarser intervals than in the image signal, may be obtained before the image signal is obtained, and the calculation of the dynamic range and the setting of the rate of dynamic range compression may be carried out in accordance with the preliminary read-out image signal.
Further, the image reproducing method and apparatus in accordance with the present invention should preferably be modified such that the image signal may be converted into a luminance, an unsharp image signal, which represents an unsharp image of the image represented by the luminance, may be formed, and the calculation of the dynamic range, the setting of the rate of dynamic range compression, and the dynamic range compressing process may be carried out in accordance with the unsharp image signal.
Furthermore, the image reproducing method and apparatus in accordance with the present invention should preferably be modified such that the dynamic range compressing process may be carried out by setting the number of bits of the image signal, which is subjected to the dynamic range compressing process, to be larger than the number of bits of the processed image signal. In particular, in cases where the processing is carried out in accordance with the unsharp image signal, the bit width of the signal subjected to the processing should preferably be set to be wider than the bit width of the input image signal such that an false contouring may not occur due to insufficient quantization level of the dynamic range compressed signal.
With the image reproducing method and apparatus in accordance with the present invention, the histogram of the image signal is formed, and the dynamic range of the image signal is calculated from the histogram. The rate of dynamic range compression corresponding to the region of the image signal, in which the signal value is comparatively large, the region of the image signal, in which the signal value is comparatively small, and/or the entire region of the image signal is set in accordance with the dynamic range and by taking the predetermined reference level of the image signal as reference. The dynamic range compressing process is then carried out on the image signal by using the rate of dynamic range compression having thus been set. The region of the image signal, in which the signal value is large, corresponds to the highlight in the reproduced image, and the region of the image signal, in which the signal value is small, corresponds to the shadow in the reproduced image. Therefore, in cases where the rates of dynamic range compression are set such that the dynamic range compressing process may be carried out on only the highlight and the shadow, the details of which are apt to become imperceptible due to insufficient gradation, the dynamic range compressing process can be carried out on only the image portion, in which the large area contrast is strong. As a result, the large area contrast is weakened only for the highlight or the shadow, and the dynamic range compressing process is not carried out on the image portion, in which the large area contrast is weak. Therefore, the details of the highlight and the shadow do not become imperceptible, and the contrast of the image portion, in which the large area contrast is weak, is not weakened. Accordingly, a reproduced image having good image quality can be obtained.
As for the reference level of the image signal, the rate of dynamic range compression is set to be equal to approximately zero. Therefore, in cases where the image density of the primary object in the image is set as the reference level, the dynamic range compressing process is not carried out for the primary object. Also, the adjustment of the brightness of the entire area of the image may be carried out independently. In this manner, the function for the dynamic range compression and the function for the brightness adjustment can be separated clearly, and the correction of brightness, or the like, can be carried out easily.
Also, in cases where the dynamic range compressing process is carried out only when the dynamic range having been calculated from the histogram is larger than the predetermined threshold value, the dynamic range compressing process is not carried out when the dynamic range is smaller than the predetermined threshold value. As a result, as for an image in which the dynamic range is small and the large area contrast is weak, the dynamic range compressing process can be prevented from being carried out. Therefore, the large area contrast of the image, in which the large area contrast is weak, can be prevented from being weakened.
Further, with the image reproducing method and apparatus in accordance with the present invention, the unsharp image signal may be formed by converting the image signal into the luminance, and subjecting the luminance to filtering processing, or the like. The dynamic range compressing process may then be carried out on the unsharp image signal. In such cases, the image reproduced from the processed image signal is obtained such that the color reproducibility may be kept good, even though the brightness at the edge of an object in the image may become different from the brightness in the color image. Therefore, an image can be obtained which is free from an unnatural feeling in comparison with the original color image.
Furthermore, in cases where the bit width of the image signal, which is subjected to the dynamic range compressing process, is set to be larger than the bit width of the processed image signal, a false contouring can be prevented from occurring due to insufficient quantization level, and an image having good image quality can be obtained.